The present invention generally relates to an apparatus and method for dispensing fluids and more specifically, to a gun driver diagnostic circuit for a fluid dispensing apparatus.
Pneumatic and electric fluid dispensers have been developed for dispensing applications requiring a precise placement of a fluid. Pneumatic dispensers have a significant advantage in that the pneumatic solenoid operating the dispensing valve provides a sufficient force so that the dispensing valve operation is essentially independent of the viscosity of the fluid being dispensed. However, pneumatic solenoids have disadvantages in that they often have a shorter life than electric solenoids, and the operation of the pneumatic solenoid is subject to less precise control than the electric solenoid in an electric fluid dispenser. Therefore, in some applications, electrically operated fluid dispensers are preferred over pneumatic fluid dispensers.
Generally, electrically operated fluid dispensers include an electromagnetic coil surrounding an armature that is energized to produce an electromagnetic field with respect to a magnetic pole. The electromagnetic field is selectively controlled to open and close a dispensing valve by moving a valve stem connected to the armature. More specifically, the forces of magnetic attraction between the armature and the magnetic pole move the armature and valve stem toward the pole, thereby opening the dispensing valve. At the end of a dispensing cycle, the electromagnet is de-energized, and a return spring returns the armature and valve stem to their original positions, thereby closing the dispensing valve.
With both pneumatic and electric dispensing guns, a driver circuit provides an output signal to an inductive load, either a solenoid coil in a pneumatic gun or a gun-operating coil in an electric gun. Changing electrical characteristics of the load or output circuit connected to the driver circuit may result in inconsistent and improper operation of the coil being driven by the driver circuit and hence, the operation of the fluid dispensing gun is adversely affected.
For example, the output circuit may be improperly wired or, wires may be damaged such that a low resistance load is connected on the driver circuit. Many driver circuits are made to withstand short circuit conditions and therefore, are not damaged by an excessively low resistance or impedance load. However, while the driver circuit may not be damaged, it is useful to be able to detect and signal the user of the above low impedance conditions so that corrective action can be taken.
In other situations, the driver circuit may be connected to a dispensing gun with a coil having an impedance that is mismatched to the output impedance of the driver circuit. While some impedance mismatching is acceptable, excessive impedance mismatching can result in an inconsistent operation of the fluid dispensing gun.
Further, over a prolonged operating period, heat in the dispensing gun can cause wire insulation to slowly degrade. Such degradation can cause windings within the coil to arc and degrade into a short circuit condition. As coil windings degrade, the resistance and inductance of the coil decreases, and the impedance is further reduced. That process may adversely affect the operation of the dispensing gun.
Consequently, there is a need for a diagnostic circuit for use with a driver circuit of a fluid dispensing gun that detects and alerts a user to lower output circuit impedances that can may either immediately, or over a period of time, adversely effect the operation of the fluid dispensing gun.
The present invention provides a simple and reliable diagnostic circuit for a driver output circuit of a fluid dispensing gun that is sensitive to a wide range of low impedance conditions. The diagnostic circuit of the present invention is capable of signaling the user of deteriorating electrical characteristics in the driver output circuit that may result in an inconsistent operation of the fluid dispensing gun. The diagnostic circuit of the present invention also signals the user in the event the user attempts to use a dispensing gun presenting a severe impedance mismatch with the output of the driver circuit. The diagnostic circuit of the present invention is especially useful in providing signals, in a timely manner, that apprise the user of conditions that may result in an improper or inconsistent operation of the dispensing gun. Thus, the user can address the condition in a timely manner and reduce the production of defective or scrap product.
According to the principles of the present invention and in accordance with the preferred embodiments, the invention provides an apparatus for detecting a low impedance load on the output of a driver circuit. The driver circuit has a power switching circuit providing output signals to a coil. Each output signal results from a drive signal that has a waveform comprising a higher magnitude initial peak period followed by a lower magnitude hold period. The apparatus comprises a diagnostic circuit connected to the driver circuit. The diagnostic circuit provides a low impedance error signal in response to detecting an output signal oscillation that crosses a signal threshold during a corresponding drive signal.
In one aspect of the invention, the driver circuit has a feedback circuit providing a difference signal as a function of a difference between an output signal and a corresponding drive signal, and the diagnostic circuit has a comparator responsive to the difference signal for producing a low impedance error signal in response to detecting an output signal oscillation that crosses a magnitude of the reference signal.
In another aspect of the invention, the diagnostic circuit has a first circuit connected to the driver circuit for providing a gating signal representing a higher magnitude initial peak period of a drive signal. A second circuit is connected to the comparator and the first circuit for producing the low impedance error signal during only the higher magnitude initial peak period of the drive signal.
In another embodiment of the invention, a method is provided for detecting a low impedance load on an output of a driver circuit that is connected to a coil. The driver circuit applies output signals to the coil in response to corresponding drive signals. Each drive signal has a waveform with a higher magnitude initial peak period followed by a lower magnitude hold period. An error signal is provided in response to detecting an output signal oscillation that crosses a signal threshold during a corresponding drive signal.
In one aspect of this invention, the error signal is provided in response to detecting an output signal oscillation that crosses a signal threshold during a higher magnitude initial peak period of the corresponding drive signal.
These and other objects and advantages of the present invention will become more readily apparent during the following detailed description taken in conjunction with the drawings herein.